Homopolymerization of acetylene substituted polyimide polymers

ABSTRACT

Polyimides are prepared from acetylene substituted polyimide oligomers via an addition polymerization reaction which involves homopolymerization. These polymers exhibit low void content when cured and possess superior thermal stability characteristics and physical properties such as structural strength. One of their unique properties is their ability to be processed into useful articles at moderate pressures and temperatures.

The invention herein described was made in the course of or undercontract or subcontract with the U.S. Air Force.

CROSS-REFERENCE TO RELATED APPLICATIONS

This invention discloses .Iadd.and claims .Iaddend.a novel method offorming cured polymers from the oligomers disclosed in application Ser.No. 347,501, filed on Apr. 3, 1973, now abandoned.[...]. .Iadd.in favorof U. S. Application S/N 413,473, filed November 6, 1973. It is aContinuation-in-Part of U. S. Application S/N 413,473, which is aContinuation-in-Part of U. S. Application S/N 347,501. .Iaddend.

.Iadd.U. S. Application S/N 413,473 subsequently issued as U. S. Patent3,845,018 on October 29, 1974. .Iaddend.

BACKGROUND OF THE INVENTION 1. Field of the Invention

Our invention deals with high molecular weight, high strength polymersfrom acetylene substituted polyimide oligomers and methods for formingsuch polymers.

2. Description of the Prior Art

The best known polyimide art deals with the production of hightemperature polyimide polymers from polyamic acids by condensationpolymerization reactions. Such reactions liberate volatile byproductssuch as water during cure and consequently produce void-filledfabricated structures when the prepolymers are used to produce suchitems as laminates. Examples of this art are the products formed frompolyamic acids and other comonomers.

The known art also describes addition copolymers of cyclopentadiene andbis maleimides. These materials cure through addition, but they tend toliberate flammable cyclopentadiene when heated above 550°F which resultsin a degradation process.

We have disclosed, in application Ser. No. 416,483 filed on Nov. 16,1973, in the names of Norman Bilow, Abraham L. Landis and Leroy J.Miller as inventors, copolymers of terephthalonitrile N,N'-dioxide andacetylene substituted polyimide oligomers which solve many of theproblems associated with the production of voids during cure andpostcure of polyimide polymers at high temperatures. Voids areundesirable because they tend to detract from the physical propertiesand thermal oxidative stability characteristics of these polymers. Ourpresent invention is a significant improvement over that process. Theability to form useful polymeric materials by homopolymerizationeliminates the need for critical prepolymer formulation requirements,and the cured end products exhibit superior thermal as well as physicalproperty characteristics.

There have been prior art disclosures which teach that bis maleimidescan be homopolymerized but the cured products are brittle. Bismaleimides can also be copolymerized with amines to give useful additiontype copolymers. One example is the commercial Kerimid 601 polyimidemanufactured by Rhodia, New York a subsidiary of Rhone Poulene. TheKerimide polyimides are reported to exhibit flexural strength up toapproximately 60,000 psi and are stable up to approximately 480°F. Themanufacturer does not recommend the use above 500°F. Our homopolymerizedpolyimides can exhibit flexural strengths of over 1 million psi in 181 Eglass fabric reinforced laminates and over 250,000 psi in unidirectionalgraphite reinforced laminates. The resins have been shown to bethermally stable for extended periods (1,000 hrs.) above 550°F in air or500 hrs. at 600°F.

SUMMARY OF THE INVENTION

We have discovered that acetylene-terminated polyimide oligomers, havingthe structure ##STR1## where M is --C.tbd.CH or ##STR2## and wherein Qis O, CH₂, SO₂, --CF₂ --_(n), CO, or S where

X=CO, O, CH₂, SO₂ or --CF₂ --_(n) and

R=C₆ H₄ --Y--C₆ H₄ --, --C₆ H₄ --Y--C₆ H₄ --Y--C₆ H₄ --, or --C₆ H₄--Y--C₆ H₄ --Y--C₆ H₄ --Y--C₆ H₄ --

where Y=CO, O, CH₂, SO₂, or CF₂ or any combination thereof if more thanone Y is present, and where ##STR3## can be replaced by ##STR4## andwhere the R group may bear one or more groups selected from the groupchlorine, methyl, phenyl or ethynyl, can be homopolymerized to formpolymers which exhibit substantially superior thermal stability,oxidative stability and high temperature flexural strength. The reactionwill proceed with or without the addition of a catalyst; however,without a catalyst temperatures of about 350°-500°F are generallyrequired. The cured polymers (resins) cured under these conditions areessentially void free especially if they are molded under moderatepressure such as 100-300 psi.

These polymers have substantial advantages in the preparation oflaminates and filled molded compositions. Because of their highstructural strength, they would be excellent for fabricating spacecraftantennas, jet vanes and various aircraft structural members.

DESCRIPTION OF THE INVENTION

In practicing our invention, we utilize acetylene-terminated polyimideoligomers of structure I. The oligomers can be dissolved in a solventsuch as N-methylpyrrolidene, sulfane, dimiethyl sulfone anddimethlformamiide to produce a varnish suitable for coating areinforcement such as glass-, carbon-, or graphite-fabric. Cutouts fromthese fabrics can be stacked and molded under pressure using a hydraulicpress, autoclave, vacuum bag, or heavy weight. The required pressuresare frequently less than 200 psi. It also is possible to add fillermaterials to the resins before molding to achieve specific properties.Typical fillers which could be used include but are not limited toclays, talc, alumina, silica sand, glass-, graphite-, orphenolic-microballons, minerals and chopped fabrics.

The oligomers cure without catalysts in the range of 350°-500°F althoughwe currently prefer the 385°-425°F range. Lower temperature cures arefeasible, but catalysts may be required to achieve useful polymerizationrates. Catalysts which may be useful include the Ziegler type catalystssuch as trialkylaluminum-titanium tetrachloride mixtures, Friedel Craftscatalysts, metal carbonyls, tetraphenyl tin, zinc and tin octanoates.These agents have been used in the past to promote acetylenictrimerizations and may be useful to promote polymerization such as ours.

Polymerization may be visualized as proceeding according to the generalreaction scheme shown in plate II wherein R is ##STR5## wherein Z is O,CH₂, S, CO, SO₂, --CF₂ --_(m), --CF₂ --O--CF₂ --

wherein n is 0-5

wherein m is 0-5

wherein R' is O, S, CH₂, SO₂, --CF₂ --_(m), --CF₂ --O--CF₂ -- ##STR6##wherein X is O, CH₂, S, CO, SO₂, --CF₂ --_(m), --CF₂ --O--CF₂ --

wherein n' is 0 to 5

wherein n" is .[.0.]. .Iadd.1 .Iaddend.to 5

Specific illustrations of methods of practicing our invention arepresented in the following examples:

EXAMPLE 1 Preparation of Oligomer 1

To a solution of benzophenonetetracarboxylic dianhydride (55.1 grams,0.171 mole) in dry dimethylformamide (215 ml) heated at reflux, asolution of 1,3-di(3-aminophenoxy)benzene (24.95 grams, 0.0855 mole) indry dimethylformamide (215 ml) was added dropwise over a 30-minuteperiod. The reaction mixture was stirred at reflux for 2 hours and thena solution of 3-aminophenylacetylene (20.0 grams, 0.171 mole) indimethylformamide (100 ml) was added over a 15-minute period and thesolution heated at reflux for an additional hour after the addition. Thesolvent was removed with the rotary evaporator and to the residue wereadded m-cresol (250 ml) and benzene (100 ml). The mixture was heated tototal reflux using a Dean-Stark water trap. After 18 hours, a total of3.2 ml of water was present in the trap. The benzene/m-cresol mix wasremoved with the rotary evaporator. The residue was extracted withseveral liters of benzene and the extract washed with dilute aqueoussodium hydroxide and water. Upon removal of the benzene with the rotaryevaporator, a gummy solid (10 grams) was obtained. The benzene-insolubleportion (70 grams) was dried in vacuum at 80°C. The product softened at190°C and upon heating at 250°C in air cured into a tough transparentamber solid.

Laminate from Oligomer of Example 1

This resin was applied to 181E glass cloth fabric (with an A-1100finish) using a hot dip method to maintain the resin in solution. Thus,the resin was dissolved in hot dimethylformamide, placed in a dip tankand coating was continued until most of the varnish was gone.

The impregnated cloth was vacuum-dried at 272°F for 16 hours and thefabric appeared to be evenly coated. A seven-ply laminate was molded at535°F and 200 psi for 4 hours. No outgassing was observed during themolding. The laminate had an excellent appearance. It was then postcuredin air using the following cycle: 4 hours at each temperature, 450°F,500°F, 550°F and 600°F. During the post-cure, the laminate lost 0.31percent in weight to yield a final void content of 1.27 percent.

EXAMPLE 2 Preparation of Oligomer 2

To a solution of benzophenonetetracarboxylic dianhydride (82.3 grams,0.256 mole) in dry dimethylformamide (325 ml) heated at reflux, asolution of 1,3-di(3-aminophenoxy)benzene (37.4 grams, 0.128 mole) wasadded dropwise over a 1 hour period. The mixture was heated at refluxfor 11/2 hour and then a solution of 3-aminophenylacetylene (30.0 grams,0.256 mole) in dimethylformamide (150 ml) was added and the solutionheated at reflux for an additional 11/2 hours after the addition. Thesolvent was removed on the rotary evaporator and to the residue wereadded m-cresol (350 ml) and benzene (150 ml). The mixture was heated tototal reflux using a Dean-Stark water trap. After several hours, 1.5 mlof water along with 50 ml of benzene were removed from the trap. Themixture was heated at total reflux overnight during which time anadditional 3.5 ml of water was collected. The benzene/m-cresol mix wasremoved with the rotary evaporator. The residue was thoroughly washedwith absolute ethanol and dried in a vacuum oven overnight at 80°C. Thedried product weighed 134.3 grams.

Laminate from Oligomer 2

A lacquer of oligomer 2 in dimethylformamide, having a solids content of18 percent, was used to impregnate 181E glass cloth with an A-1100finish. The impregnated cloth was dried in a circulating air oven at325°F for 15 minutes and then vacuum dried at 250°F for 4 hours. Aseven-ply laminate was molded at 525°F, 200 psi, 2 hours, using acontact time of 90 seconds prior to molding. The resulting laminate hada resin content of 32.8 percent. It was then post-cured in air using thefollowing cycle: 4 hours at each temperature, 450°F, 500°F, 550°F and600°F. There were several small blisters present on the surface.Specimens were cut from the areas which did not show any blistering. Thevoid content of this laminate was 3.5 percent.

EXAMPLE 3 Preparation of Oligomer 3

To a solution of benzophenonetetracarboxylic dianhydride (85.0 grams,0.246 mole) in dry dimethylformamide (325 ml) heated at reflux, asolution of 1,3-di(3-aminophenoxy)benzene 38.5 grams, 0.132 mole) indimethylformamide (3245ml) was added dropwise over a 2 hour period. Themixture was heated at reflux for 60 minutes and then a solution of3-aminophenylacetylene (31.4 grams, 0.264 mole) in dimethylformamide(150 ml) was added. The mixture was heated at reflux for 30 minutes. Thesolvent was removed on the rotary evaporator and to the residue wereadded m-cresol (350 ml) and benzene (150 ml). The mixture was heated tototal reflux using a Dean-Stark water trap over a weekend during whichtime 4.8 ml of water was trapped. The benzene/m-cresol mixture wasremoved with the rotary evaporator. The residue was thoroughly washedwith absolute ethanol and dried in a vacuum oven for 24 hours at 80°C.The dried product weighed 136.8 grams.

Laminate from Oligomer 3

A lacquer of oligomer 3 in dimethylformamide, having a solids content of15 percent, was used to impregnate 181E glass cloth (with an A-1100finish). The impregnated cloth was dried in a circulating air oven at325°F for a few minutes and then vacuum dried at 272°F for 16 hours. Aseven-ply laminate was molded at 485°F, 200 psi, 2 hours, using acontact time of 150 seconds. The resulting laminate had a resin contentof 44.8 percent. It was then post-cured in air using the followingcycle: 4 hours at each temperature, 450°F, 500°F, 550°F and 600°F. Anexcellent molded laminate resulted which had a final void content of 1.2percent. During the post-cure the laminate lost less than 0.5 percent ofits weight.

We claim:
 1. A cured resin derived by the homopolymerization ofacetylene substituted polymide oligomer having the following generalstructure: ##STR7## wherein R is ##STR8## wherein Z is O, CH₂, S, CO,SO₂ --CF₂ --_(m), or --CF₂ --O--CF₂ --, andwherein n is 0-5, m is 0-5,and wherein R' is O, S, CO, CH₂, SO₂, --CF₂ --_(m), or --CF₂ --O--CF₂--, and wherein X is O, S, CH₂, CO, SO₂ --CF₂ --_(m), or --CF₂ --O--CF₂--, andwherein n' .[.is 0.]. .Iadd.average about 1 .Iaddend.to 5 and n"is 0 to
 5. 2. A resin of claim 1 wherein R is as defined above X is CO,n' is 0, R' is 0 n"is
 1. 3. The process of producing cured resins ofclaim 1 comprised of heating said acetylene substituted polyimideoligomer at elevated temperatures ranging from 190°-600°C at a pressurefrom about 15 psi to about 200 psi.
 4. The process of producing curedresin of claim 1 comprised of heating acetylene substituted polyimideoligomer at elevated temperatures in the presence of catalyst taken fromthe group comprosed of Ziegler catalyst, Friedel Craft catalyst or nonFriedel Craft organo metallic catalyst. .Iadd.
 5. A cured resin as setforth in claim 1, wherein said resin is incorporated as part of alaminate. .Iaddend. .Iadd.6. A cured resin as set forth in claim 2,wherein said resin is incorporated as part of a laminate. .Iaddend..Iadd.7. The process as set forth in claim 3, wherein said heating ofsaid oligomer is accomplished after said oligomer is incorporated aspart of a laminate. .Iaddend. .Iadd.8. The process as set forth in claim4, wherein said heating of said oligomer is accomplished after saidoligomer is incorporated as part of a laminate. .Iaddend.